im looking at buying a 1997 f250 4x4 7.3 powerstroke diesel truck it has 166000 miles and was a plow truck before it looks like. i went and checked it out it started up real nice and sounded good i got to driving it down the road and the truck runs and get up to speed good but when u do a constant speed the truck makes this whistling or this hissing sound i could figure out what it was so does anyone have any ideas. i like the truck but that sound scars me and its kinda loud]any advice would be very greatful guys thanks
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I have a 97 crew stroke myself and with the muffler gone and straight piped the turbo whistle is there at constant speeds. If it has a boost gauge you could have a leak in that as the line would come up into the cab.

I have a early 99 PSD. Mine does the same when cold, has since i bought it in 2004. Drive it for a good ten minutes at speed, i bet it goes away once good and warmed up. Been told it's wastegate (turbo) related but I'm not sure on that, but regardless it hasn't been a issue the last 8 years other than the loud noise until warmed up good. Buy it up!

It probably has a turbo leak and when cold the ebv is closed not allowing boost to build to rapid heat the motor to operating temp faster. Mine was a pita so I gutted the pedestal and removed the valve and actuator.

Was it gutless as hell until it warmed up or you drove it for a few minutes?

Try a longer drive. Mine acts just as you describe every cold morning ( about 40-45&#176; or less). Has no noticeable lack of power, as it goes away once you jump on the throttle or let off completely. Only hisses while holding steady. That being said i do believe its not suppose to act that way, but has not been a issue.

Maybe the vacumn pump leaking? How was the brakes? If you got good power, and no smoking...I wouldn't worry to much. If its a concern ask to take it in for a used car inspection at a local shop. I do them all the time at my shop. Usually $25 or so.

I had my ebpv staying closed last winter It sounded like blowing compressed air through a 4" pipe from idle or when I was on it. I pulled it out and now I just have the beautiful turbo whistle.

A few other things I check for on powerstrokes: pop off the intake boot to check shaft play on the turbo, I'd make sure the motor is cold to the touch on startup (a lot of issues won't surface unless it's been sitting overnight), make sure the oil pan isn't rusted, and make sure there is no melted or electric burning smell around the 4 valve cover harness plugs. Like someone else mentioned taking it to a good mechanic for a once over isn't a bad idea either. Good luck!